It is well known, particularly in the field of transporting and using particulate materials, commonly powders, granules, pellets, and the like that it is important to keep product particles as free as possible of contaminants. Particulates are usually transported within a facility where they are to be mixed, packaged or used in a pressurized tubular system that typically produces a stream of material that behaves somewhat like a fluid and, thus, can be conveyed pneumatically through pipes. As these materials move through the pipes, considerable friction is generated not only between the particles themselves (referred to as internal friction), but also between the tube walls and the particles in the stream (referred to as wall friction).
In high velocity dilute phase systems in which the velocity is in the range of 25-40 meters/second, this friction results in the development of particle dust, broken particles, fluff, streamers (ribbon-like elements that can “grow” to become comparatively long and tangled), and glass fibers in glass filled products, that can impede the flow of materials or even totally block the flow of material through the pipe. The characteristics of such a transport system are quite well known, as is the importance and value of keeping product particles as free as possible of contaminants. In slow motion dense phase systems, the velocity is low (in the range of 2-10 meters/second), but the pressure is high (in the range of 0.5 to 3.5 bar). This results in high wall friction and friction between the pellets compressed in plugs moving through the pipe. This high friction creates very fine, high electrostatic-charged dust within the product being conveyed.
The term “contaminant” as used herein includes a broad range of foreign material and includes foreign debris as well as broken particles or streamers formed from the product being transported through the pipe. In either case, using plastics as an example, such foreign debris would have a detrimental effect on the finished product. Specifically, foreign debris, which by definition is different in composition from the primary material, and would include material such as dust, and non-uniform material of the primary product, such as streamers, would not necessarily have the same melting temperatures as the primary product being conveyed and would cause flaws when the plastics material is melted and molded. These flaws result in finished products that are not uniform in color, may contain bubbles, and often appear to be blemished or stained, and are, therefore, unsalable.
It is important also to note that since these same non-uniform materials often do not melt at the same temperature as the primary product, the unmelted contaminants cause friction and premature wear to the molding machines, resulting in downtime, lost production, reduced productivity, increased maintenance and thus increased overall production costs. Streamers can create problems throughout the conveying system, and in the manufacturing processes, but can also reduce or clog the discharge system to the scale, which results in errors in the weighing process. Long streamers and fine micro dust are very difficult to remove from the product. A conveying system with medium velocity (in the range of 15 to 25 meters per second) and medium pressure (in the range of 0.5 to 2.0 bar) would not have a tendency to create long streamers and fine micro dust or the other extreme contaminates created by the dilute and dense phases of conveying particulate material.
Dust, streamers and other contaminants are generated mostly by the transport system. Accordingly, it is of primary importance to not only provide apparatus that provides for a thorough cleaning the particulate material being conveyed, but to do so as close to the point of use of the particulate material as possible so as to avoid the generation of contaminants through additional transport. For these reasons, compact dedusting devices have been used for many years to clean materials in such applications. The compact dedusting devices are capable of handling smaller volumes of product, yet also capable of thoroughly cleaning the product. The compact dedusting devices permit the installation of the dedusting device immediately before final use of the products, rather than at an earlier stage after which re-contamination can occur within the conveying system.
Dedusting devices used to clean contaminants from particulate material can be found in U.S. Pat. No. 5,035,331, granted to Jerome I. Paulson on Jul. 30, 1991, in which air is blown upwardly through wash decks over which a flow of contaminated particulate material is passed so that the flow of air up through the wash decks removes the contaminants from the material flow. A magnetic field is provided by the dedusting device so that the particulate material flow passes through the magnetic field to neutralize the static charge on the particulates and facilitate the removal of the contaminants from the material. The flow of contaminant laden air is discharged from the dedusting device, while the cleaned particulate material is passed on to the manufacturing process.
A compact dedusting apparatus is disclosed in U.S. Pat. No. 6,595,369, granted on Jul. 22, 2003, to Jerome I. Paulson. Like the larger dedusting apparatus depicted in U.S. Pat. No. 5,035,331, the flow of particulate material through the dedusting apparatus is cleansed of contaminates that have had the static charged attracting the contaminates to the particulates neutralized by a magnetic flux field. The cleaning process also utilizes a flow of air passing through the stream of particulate material passing over wash decks. The contaminate-laden air is discharged through the top of the dedusting apparatus, while the cleaned particulate material is discharged from the bottom of the dedusting device.
Conventional pneumatic conveying systems would provide conveying of particulate material in either a dense phase or in a dilute phase. In the dense phase, the particulate material moves through the pipe rather slowly in a packed, though fluidized, state. Dense phase systems move more product per pound of air, but travel at lower velocities and at higher pressures. Typically, system operating pressures will not exceed 3.5 bar (50 psig). Dilute phase conveying systems, however, utilize a high velocity stream of air contained within the pipe. Dilute phase velocities often exceed 35-40 meters per second (5000-8000 feet per minute) and use up to one pound of air to move as much as five pounds of product through the pipe. Typical air pressures for dilute phase systems will be approximately 0.8 bar (about 12 psig). High air velocity and low product particle population is accomplished with low pressure resistance, but greatly increased damage to the product particles being conveyed. Damage occurs in straight pipe sections, but is greatly increased whenever directional changes are imposed on the conveying system.
Turn-down ratios in terms of pneumatic conveying of particulate material can be defined as the reduction of the rate of flow of product through the conveying pipes as compared to the designed flow rate. For example, if a pneumatic conveying system is designed to convey 100 tons of particulate product per hour, and the operator desires to reduce the rate to 50 tons per hour, adjustments have to be made in the parameters for conveying the product. The dilute and dense phases of conveying have little flexibility in turn-down rations. Reducing the flow rate in a dense phase conveying system will likely result in plugging the conveying pipes with particulate material. Simply reducing the ration of product to air in a dilute phase conveying system can result in substantial damage to the product being conveyed. An intermediate conveying phase, referred to as a Strandphase® conveying, having velocities and pressures between the dilute and dense conveying phases, is more flexible in adapting to turn-down ratios.
In pneumatic conveying systems, whether the conveying system is operating under dilute phase or dense phase, the product particles suffer considerable damage during transport, particularly when changes in direction are being used. Therefore, when changes in direction of the pipe through which the particulate material are being conveyed are required, elbow fittings are utilized. Elbow fittings for pneumatic conveying systems, in order to effect changes in direction, will often have a radius as much as ten times the diameter of the pipe being utilized. Even with such elbow fittings, the combination of high velocity and centrifugal force does most of the damage to the particulate material, particularly with respect to heat sensitive plastic compounds.
Elbow fittings used in pneumatic conveying systems typically suffer wear at the elbow curve in line with the product flow into the elbow fitting, whether the elbow fitting is a short radius elbow or a long radius sweep elbow fitting. Particles flowing into the elbow fitting impact the curved surface of the elbow and are redirected. The bouncing product particles create an area of turbulence that slows the speed of conveyance of the particles through the system. Furthermore, the bouncing particles and the movement of the product particles around the outer surface of the elbow fitting generate friction, making the surface of the fitting warm to the touch. This heat can have a detrimental effect on the product being conveyed, particularly when the product is heat sensitive, such as plastic pellets on which the edges of the pellets will melt and adhere to the pipe.
A solution to providing an elbow fitting with minimal wear characteristics can be found in U.S. Pat. No. 6,951,354, granted on Oct. 4, 2005, to Jerome I. Paulson, and in U.S. Pat. No. 7,300,074, granted on Nov. 27, 2007, to Jerome I. Paulson, both of which have been assigned to Pelletron Corporation. In these two patents, an elbow fitting expands from the inlet pipe along the outer side of the elbow fitting to define a generally triangularly shaped configuration that retains a layer of slowly moving particles being conveyed within the pneumatic conveying system along the outside surface of the fitting to deflect incoming product flow. In U.S. Pat. No. 7,300,074, a step feature was added to the outer surface structure of the elbow fitting to create a Bernoulli Effect causing the accumulated product particles to enter the air flow after the incoming flow of product particles has ceased.
Accordingly, it would be desirable to provide a new process for pneumatically conveying particulate material through pipes with medium velocities and medium pressure. Such a process would minimize damage to the particulate material while cleaning dust and debris from the particulate material to provide a high quality product to the end user. It would also be desirable to provide an improved process for conveying particulate materials used in the plastics industry.